1. Field of the Invention
The present invention relates to a process for the long-term operation of a heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid, by conducting a starting reaction gas mixture which comprises acrolein, molecular oxygen and at least one inert diluent gas through a fixed catalyst bed at elevated temperature whose catalysts are such that their active composition is at least one multimetal oxide which contains the elements Mo and V, and by, in order to counteract the deactivation of the fixed catalyst bed, increasing the temperature of the fixed catalyst bed over time.
2. Discussion of the Background
Acrylic acid is a reactive monomer which is suitable as such or in the form of its alkyl esters, for example, for preparing polymers which may find use as adhesives or water-absorbent materials, among other uses.
It is known that acrylic acid can be prepared on the industrial scale by a process for heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid, by conducting a starting reaction gas mixture comprising acrolein, molecular oxygen and at least one inert diluent gas through a fixed catalyst bed at elevated temperature whose catalysts are such that their active composition is at least one multimetal oxide which contains the elements Mo and V (cf., for example, DE-A 4431949, WO 0053559).
It is also known that such a process for heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid may be operated substantially continuously over prolonged periods over one and the same fixed catalyst bed. However, the fixed catalyst bed loses quality in the course of the operating time. In general, both its activity and the selectivity of target product formation deteriorate.
In order, despite this, to be able to operate the fixed catalyst beds, whose manufacturing and exchange is comparatively inconvenient and costly, for as long as possible in a reactor system charged with them, the prior art attempts in highly differing ways to counteract their aging process.
EP-A 990 636 (for example page 8, lines 13 to 15) and EP-A 1 106 598 (for example page 13, lines 43 to 45) propose the substantial compensation of the reduction in the quality of the particular fixed catalyst bed by gradually increasing the temperature of the particular fixed catalyst bed in the course of the operating time under otherwise substantially constant operating conditions, in order to substantially retain the acrolein conversion in single pass of the reaction gas mixture through the fixed catalyst bed.
In this context, the temperature of the fixed catalyst bed refers to the temperature of the fixed catalyst bed when the partial oxidation process is performed, except in the theoretical absence of a chemical reaction (i.e. without the influence of the heat of reaction). This also applies in this document. In contrast, effective temperature of the particular fixed catalyst bed refers in this document to the actual temperature of the fixed catalyst bed taking into account the heat of reaction of the partial oxidation. When the temperature of the fixed catalyst bed is not constant along the fixed catalyst bed (for example, in the case of a plurality of temperature zones), the term temperature of the fixed catalyst bed in this document means the (numerical) average of the temperature along the fixed catalyst bed.
It is significant in the aforementioned context that the temperature of the reaction gas mixture (and thus also the effective temperature of the fixed catalyst bed) passes through a maximum value (known as the hotspot value) when it passes through the fixed catalyst bed. The difference between hotspot value and the temperature of the fixed catalyst bed at the location of the hotspot value is referred to as the hotspot expansion.
A disadvantage of the procedure recommended in EP-A 990 636 and in EP-A 1 106 598 is that, with increasing increase in the temperature of the fixed catalyst bed, its aging process is accelerated (for example certain movement processes within the catalysts which contribute to aging proceed more rapidly). In general, this is in particular because hotspot expansion usually rises more steeply than the temperature of the fixed catalyst bed itself with an increase in the temperature of the fixed catalyst bed (cf., for example, page 12, lines 45 to 48 of EP-A 1 106 598 and page 8, lines 11 to 15 of EP-A 990 636). The effective temperature of the fixed catalyst bed therefore usually increases disproportionately in the hotspot region, which additionally promotes the aging of the fixed catalyst bed.
When a maximum value of the temperature of the fixed catalyst bed is attained, the fixed catalyst bed is therefore customarily fully exchanged.
However, a disadvantage of such a complete exchange is that it is comparatively costly and inconvenient. The process for preparing acrylic acid has to be interrupted for a prolonged time and the costs of catalyst preparation are likewise considerable.
Operating modes are therefore desired for processes for a heterogeneously catalyzed gas phase partial oxidation of acrolein to acrylic acid which are helpful in as far as possible prolonging the on-stream time of the fixed catalyst bed in the reactor system.
In this regard, DE-A 10 232 748 recommends, instead of fully exchanging the fixed catalyst bed, only replacing a portion thereof with a fresh catalyst charge.
A disadvantage of this proposal is that even a partial change of the fixed catalyst bed is accompanied by a significant cost and inconvenience.
EP-A 614 872 recommends extending the on-stream time of the fixed catalyst bed by, after operating the fixed catalyst bed for several years, which is accompanied by increases in the temperature thereof of from 15° C. to 30° C. and more, interrupting the process for partial oxidation, and, at fixed catalyst bed temperatures of from 260 to 450° C., conducting a gas mixture composed of oxygen, steam and inert gas through it, and subsequently continuing the partial oxidation.
In this context, inert gases in a gas mixture which is conducted through the fixed catalyst bed under certain conditions refers in this document to those gases of which at least 95 mol %, preferably at least 98 mol %, most preferably at least 99 mol % or 99.5 mol %, remains unchanged when they are conducted through the fixed catalyst bed. Regarding the gas mixture G to be used in accordance with the invention, steam and CO should not be included under the term inert gas.
However, a disadvantage of the procedure of EP-A 614 872 is that, up to the point at which the partial oxidation is interrupted, the aging of the fixed catalyst bed continues and is promoted unhindered.